1. Field of the Invention
The present invention relates to a method and to an apparatus for refining paper stock, and more particularly to controlling a variable speed gearmotor of a disk refiner.
2. Description of the Prior Art
Presently, optimum control of disk refiner power is, at best, a compromise between various variable parameter settings and the required main motor power levels. This compromise is the result of the fixed output shaft speed of the present gearmotors and the fact that the higher the refiner main motor power level the greater the increase in this power for a corresponding incremental change in disk position. As shown in detail below, a typical main motor drive of a disk refiner, when plotted against refiner disk clearance (decreasing clearance), shows three different response requirements of a control system, which requirements are dependent on the main motor power level, but are not variable since the output rotational speed of the gearmotor is fixed. This has led to attempts at controlling the rate of adjustment through varying the pulse frequency to the gearmotor as a function of main drive power level.
Such attempts have been unsuccessful because by their varying nature, they cause the gearmotor to operate in an inrush current condition, therefore resulting in premature gearmotor failure and excessive wear on the mechanical adjustment mechanisms.
In U.S. Pat. No. 3,309,031 R. F. McMahon et al disclose a system in which the change of temperature of material being worked is measured as the change in work absorbed by the material for adjustably controlling effect of the material working device toward a predetermined value.
John A. Gundaz in his U.S. Pat. No. 3,610,541, discloses an apparatus in which the control is to be a function of process deviation from setpoint to provide output pulses for a gearmotor which have a frequency dependent on the magnitude of deviation.